1. Field of the Invention:
This invention relates to a method of producing biaxially or multiaxially stretched ethylene-vinyl alcohol copolymer films.
2. Description of the Prior Art
Generally, in order to improve the physical properties of plastic films, biaxial stretching has been employed and different kinds of techiques have been established for various kinds of films. However, the stretching of ethylene-vinyl alcohol copolymer films is, at best, difficult and satisfactory methods for accomplishing the same are still under development. It is believed that stretching of ethylene-vinyl alcohol copolymer is difficult or the films break or rupture because the copolymer contains a large number of hydroxyl groups in its molecules which easily form hydrogen bonds during the formation of the unstretched film.
The state-of-the-art techniques for stretching ethylene-vinyl alcohol copolymer films can be divided into two categories: (A) Stretching a film of high moisture content over a low temperature range including temperatures close to the glass transition temperature of the copolymer; and (B) Stretching a film at low moisture content over a high temperature range including temperatures close to the melting point of the copolymer. However, stretching of films over low temperature ranges and at low moisture contents has been considered difficult. For example, known embodiments of the first technique involve stretching films at a temperature from the glass transition point to 150.degree. C. at a moisture content of not less than 4% (Japanese Patent Publication No. 43,199/1978), stretching films at a temperature of 60.degree. C. to 160.degree. C. at a moisture content of 8 to 30% (Japanese Patent Application Laid-open No. 15,570/1977), stretching films at a temperature of 40.degree. to 140.degree. C. at a moisture content of 8 to 20% (Japanese Patent Application Laid-open No. 30,670/1978) and stretching films at a temperature of 50.degree. to 100.degree. C. at a moisture content of 5 to 20% (Japanese Patent Application Laid-open No. 129,777/1977). Embodiments of the second technique include stretching films at a temperature of 150.degree. C. to a temperature lower than the melting point by 5.degree. C. at a moisture content of not more than 4% (Japanese Patent Publication No. 43,198/1978) and so on. In addition to the above described techniques, a special technique is also known in which a multilayer film which contains said copolymer layer laminated with a layer of easily stretchable thermoplastic resin is stretched.
Water can act as a plasticizer and prevent hydrogen bond formation between ethylene-vinyl alcohol copolymer molecules, making stretching easier, while at higher temperatures, hydrogen bonds are easily broken so that stretching of the film is made easy without the aid of the plasticizing effect of water. However, these methods are insufficient in increasing the molecular orientation of the film by stretching. It is believed that the unsatisfactory characteristics of ethylene-vinyl alcohol copolymer films relative to the dependency of the gas barrier properties upon humidity as well as water resistance and mechanical properties can be improved by increasing the orientation of the film molecules as well as increasing the degree of crystallinity of the film. Therefore, in order to increase the molecular orientation of the film to the highest degree, stretching at lower temperatures and at low moisture contents is desirable. However, such stretching has been difficult in the prior art.
Furthermore, with respect to oriented films formed with the aid of the plasticizing effect of water or high temperatures, the effect of orientation is disadvantageously not sufficiently revealed, and when a laminated film is stretched the process is complicated and is not economic. If the degree of orientation imparted to the film by stretching is insufficient, the extent of improvement in physical properties such as water resistance, blocking temperature, impact resistance, mechanical strength at low temperatures and gas barrier characteristics under high humidity conditions is not satisfactory and for practical purposes, the resulting film is not satisfactory.
Ethylene-vinyl alcohol copolymer films possess excellent gas barrier characteristics and eminently fulfill the function in the form of a unilayer film or laminated film of preserving food and medicines when used as a packaging material. However, commercially available unstretched ethylene-vinyl alcohol copolymer films possess unsatisfactory water resistance, blocking temperature and impact resistance and possess unsatisfactory mechanical properties at low temperatures. Moreover, the gas barrier characteristics are variable and are dependent on humidity with yet a further drawback being that the gas barrier characteristics significantly decrease under high humidity conditions. Therefore, a solution to these problems has been awaited most anxiously in order to increase the range of use of the film, and a need continues to exist for a method of improving the orientation of ethylene-vinyl alcohol films by a stretching process.